This invention relates to organic electroluminescent (EL) devices comprising a light emitting layer (LEL) containing a host and a dopant wherein the dopant comprises a 2-(3-aminophenyl)-benzofuran or 2-(4-aminophenyl)benzofuran compound.
While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, xe2x80x9cDouble Injection Electroluminescence in Anthracenexe2x80x9d, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 xcexcm). Consequently, operating voltages were very high, often  greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g.  less than 1.0 xcexcm) between the anode and the cathode. Herein, the term xe2x80x9corganic EL elementxe2x80x9d encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
A common dopant used in the LEL is 2,5,8,11-tetra-tert-butylperylene (TBP). In addition, light emission in the blue color region has been shown, such as in US2002-041976, JP2001-196182, JPO5-109485, and JP2000-192028, by using a non-aminophenyl substituted benzofuran in the LEL. Notwithstanding the foregoing developments, there are continuing needs for organic EL device components that can further increase blue luminance and luminance yield.
The invention provides an organic light emitting diode device containing a light emitting layer (LEL) comprising a host and a blue light emitting dopant wherein the dopant comprises a 2-(3-aminophenyl)-benzofuran or 2-(4-aminophenyl)-benzofuran compound and an imaging device containing the same. Also provided is a light emitting device, and a static or motion imaging device, and a method of emitting light.
Dopants of the invention enable improved luminance and luminance yield.